The government is investigating another possible case of mad cow disease, the Agriculture Department said yesterday.

Testing indicated the presence of the disease in a cow that died on the farm where it lived, said John Clifford, the department's chief veterinarian. The agency would not say where the farm was. The cow was at least 12 years old and died of complications during calving, Clifford said.

"It is important to note that this animal poses no threat to the human food supply, because it did not enter the human or animal food chains," Clifford said.

The department is conducting further tests and is sending a sample of brain tissue to a lab in Weybridge, England, Clifford said.

Two other cases of the disease have been confirmed in the United States, in a Texas cow that died in November and in a Canadian-born cow detected in December 2003 in Washington state.

In the latest case, the cow died on the farm where it lived, and a private veterinarian removed brain tissue for sampling, Clifford said.

But testing options are limited in this case. Because the farm was remote, the private veterinarian who removed a tissue sample used a substance to preserve it. The animal died in April, but the veterinarian forgot to send the sample to the Agriculture Department until this month, Clifford said.

Use of the preservative means that two tests commonly done when mad cow disease is suspected, initial rapid screening and western blot, cannot be performed on this sample, he said. The sample will be tested using immunohistochemistry, which produced conflicting results on the Texas cow.

The fatal brain-wasting disease is known medically as bovine spongiform encephalopathy, or BSE. Human consumption of meat products tainted with BSE has been linked to about 150 deaths from a fatal disorder called variant Creutzfeldt-Jakob disease.